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Operation Details
Switching devices can sometimes have a leakage current that exceeds the
IT
min
device, an impedance (typically, a resistor) needs to be used in parallel with
the input.
For example, some of the newer proximity switches use two wires instead of
three. The third wire was used for a power or ground line. Without the third
wire, the switch is easier to install. However, it requires more leakage current
in the off state to power its internal circuitry.
As a conservative estimate, use the following formula to calculate an exter-
nal resistance value. It keeps the input voltage at or below 2.4V when the
switching device is in the "off" state.
If the switch leakage specification is ≤ 1.7 mA, then:
Use a resistor less than or equal to 2.5 KΩ. Be sure that the wattage is ade-
quate for the resistor when the switching device is in the "on" state remem-
bering that:
P
A.3
DC Outputs
A DC Output consists of a solid state switch. It turns on or off according to the logic
state sent to it by the application program. If the application program sends it a logic
1, the switch closes and the device is powered. If the application program sends a
logic 0, the switch opens and power to the device is cut off. The application program
updates the logic state for each switch every time it scans the program.
The logic side of the switch is optically isolated from the field side. If you need to know
whether voltage is actually present at the field side, use a voltmeter.
A.3.1
Inductive Loads
Resistive and inductive loads can be connected to DC outputs and controlled by the
system with no precautions other than making sure they have a connection to the
common of the DC power supply.
Inductive loads have an electrical "kickback" when current is stopped. This can
damage or destroy the output switching device. Each output has a diode through
which reverse current can be safely routed.
The internal diode works with an inductive load. When an output is energized,
represented by a closed switch, current passes through the load into the common line.
When the output is de-energized, represented by an open switch, current stops and
the inductive field collapses. The state of the outputs is controlled by the CPU. This
creates a reverse voltage across the load called "kickback" which tries to continue the
174
(current allowed when off) of an input module. In order to use such a
2.4V
-------------------------------------------------------------- -
Switch Leakage 0.75mA
–
2.4V
≥
------------------------------- -
2.5KΩ
1.7 0.75mA
–
2
V
ON
=
--------- -
R
version 15.1
IMPORTANT
≥
R
Standalone MMC Hardware Manual
Danaher Motion
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